Currently, custom models for custom items are supported, but the Editor provides little help with actually making those models: the users have to use a third party model editor like Blockbench or the Mr Crayfish model creator to create those models. Also, the user will have to do manual modifications to the JSON files generated by these programs to adjust them for the Editor.

This situation could be improved by including a slightly modified version of one of these two in the Editor. Luckily for us, both of them are open source. Including the Mr Crayfish would be easiest because it is a plain runnable JAR file (just like the Editor itself). Blockbench is an Electron app and is thus probably harder to include.

It looks like Blockbench is getting much more popular than Clayfish, so I don't think I would go with this approach.